1. Technical Field of the Invention
The present invention relates to an image forming apparatus which is utilized for copying machines, printers, facsimiles and the like whereby an image is formed in an electrophotographic method, and more particularly, to an image forming apparatus for forming an image by scanning an image forming surface of photoconductor or the like with a deflected optical beam.
2. Description of Related Art
An image forming apparatus of this kind is provided with a scanning optical system which is arranged for scanning an image forming surface by deflecting an optical beam. The scanning optical system is generally combined with a plurality of lenses such as an f.theta. lens for enabling a deflected optical beam to perform a scanning in uniform velocity, and a cylindrical lens for rectifying surface inclination of a deflector for its normal posture.
When a surface inclination is rectified with a cylindrical lens, it is necessary to bring the cylindrical lens close to a photoconductor on which description will be made hereinafter referring to FIGS. 5 and 6.
As illustrated in FIG. 5, for instance, in the case when the surface of a polygon mirror a which is used as deflector is inclined, if a light receiving point on a bus b is neither before nor behind relative to a conjugate point e, an image point is on an optical axis, and irregularity of pitch, and generation of fuzz are prevented since an optical path A between a cylindrical lens c and the polygon mirror a, and an optical path B between the cylindrical lens c and a photoconductor d are maintained in a conjugate relation.
As shown in FIG. 6, however, when the surface of a polygon mirror a is inclined, and if a light receiving point is either before or behind relative to a conjugate point e, an optical beam f reflected by the polygon mirror a passes a position deviated by l.sub.1 from a bus b at a conjugate point e. An image is therefore formed at a position deviated by l.sub.2 from the bus b on a photoconductor d to cause irregularity of pitch and generation of fuzz.
The relation between l.sub.1 and l.sub.2 is l/f=l/A+l/B, and is l.sub.1 : l.sub.2 =A:B. When let B/A equals .beta., l.sub.2 =l.sub.1 .beta.. For restraining the irregularity of pitch, l.sub.2 needs to be lessened. In other words, either l.sub.1 or .beta. has to be lessened. l.sub.1 equals t sin .alpha. (where t is determined by the amount of movement of deflection point, distance between the axis and surface of polygon mirror, and the number of surfaces, and .alpha. is the amount of surface inclination). In order to lessen l.sub.1, the amount of surface inclination needs to be lessened. More particularly, it is necessary to process the surface of polygon mirror in high accuracy which eventually causes a rise in manufacturing cost. Since there is a limit to a the accuracy in manufacturing process, it is not suitable for a beam scanning operation in high resolution. On the other hand, if .beta. is lessened, i.e. a magnification in subscanning direction is lessened, the cylindrical lens is brought closer to the photoconductor as the magnification lessens.
The scanning optical system has heretofore been installed in a housing when it is attached to a structural member of the main body of an image forming apparatus. However, if the cylindrical lens c is brought closer to the photodonductor d as described above, the cylindrical lens c is largely separated from the polygon mirror a.
The scanning optical system thus becomes large in size in the above-mentioned conventional method of installation, and a housing in which the scanning optical system is installed eventually becomes large in size. If the housing becomes large in size, the rigidity as well as the accuracy in overall dimension and shape of the housing are lowered in the conventional manufacturing process, and it becomes difficult to accurately assemble the scanning optical system since it affects postures and, positional relationships of a plurality of lenses to be installed in the scanning optical system.
In recent years, it has been practiced to manufacture a housing of a scanning optical system with resin material in order to lower manufacturing cost, however, it becomes more difficult to accurately manufacture the housing since the rigidity of the housing is further lowered.
In order to solve such problems, it may be considered to separately hold a plurality of lenses in individual housings, and install each one of the divided housings at a predetermined position on a structural member of the main body of an image forming apparatus.
In Published Unexamined Utility Model Application No. 4-20060, there is disclosed an image forming apparatus with a construction as described above. In this apparatus, however, a plurality of lenses are simply attached to individual housings fixed on a structural member of the image forming apparatus without giving any consideration to the rigidity of the structural member. The structural member of the main body of the image forming apparatus itself thus lacks sufficient rigidity, and it eventually affects the postures and positional relationships of a plurality of lenses to invite insufficient accuracy. Further, it may cause to generate image noise since each one of the lenses disposed at different positions is affected by vibration which is generated in the main body. When a plurality of lenses are separated from the main body of an image forming apparatus, it is difficult to control optical accuracy in a scanning optical system.